1. Field of the Invention
The present invention generally relates to an apparatus for applying a gel to a plurality of optical fibers and, in particular, relates to one such apparatus having means for injecting a gel into a cavity having a plurality of separated optical fibers extending therethrough such that each of the optical fibers is coated with the gel.
2. Description of the Prior Art
In the manufacture of fiber optic cables, a plurality of optical fibers are first collected into fiber optic bundles. A number of these fiber optic bundles are then wound together to form the fiber optic cable. Typically, during the formation of the fiber optic bundles a plurality of optical fibers are passed through an extrusion cross head. As the fibers are passed through the extrusion cross head, heat is applied such that a plastic sheath is extruded around the fibers to contain the optical fibers as a bundle. One major difficulty encountered during this process is that the heat required for applying the sheath can soften the thermoplastic ink coating on the optical fibers causing the fibers to adhere, or stick, to each other. Such sticking can result in a catastrophic transmission loss of the optical fibers and thus lead to a failure of the resultant fiber optic cable.
It is generally accepted practice in the industry, therefore, to apply some form of filling compound to the optical fibers prior to passing the optical fibers through the extrusion cross head. Historically, the application of filling compounds has been largely unsuccessful.
Conventional techniques for applying a filling compound exhibit numerous drawbacks. For example, failure to maintain a sufficient level of filling compound in the application cavity or failure to maintain adequate pressure in the cavity can result in the formation of air gaps within the fiber optic bundle and thus allowing the fibers to adhere to each other. Further, when a higher pressure is applied or a high viscosity material is used, the drawing tension has increased to an unsafe level during the extrusion of the tube around the optical fibers. A further difficulty of conventional techniques is that the alignment of the output of the filling compound application unit with the extrusion cross head is crucial. If the filling compound application unit and the extrusion cross head are not precisely aligned, the optical fibers are dragged on the extrusion cross head entrance thus causing damage to the optical fibers.
Another conventional technique for avoiding the sticking of optical fibers during the formation of the bundles is the use of an oil applicator. Typically, such techniques lubricate the surfaces of the separated optical fibers with a low molecular weight oil. However, such oils often evolve off the fiber with the application of heat by the extrusion cross head. Hence, the oil dissipates before the optical fibers cool and the optical fibers stick to each other. Further, the chemical interaction between the selected oil and the thermoplastic ink coating on the optical fibers can be difficult to match to avoid sticking among the fibers. In addition, the quantity of oil entering the buffer tube is also difficult to control.
These difficulties have been substantially overcome as described in U.S. Pat. No. 5,395,557 entitled Method and Apparatus for Applying Gel to a Plurality of Optical Fibers which issued to Griser et at. on Mar. 7, 1995 and is assigned to the assignee hereof. The teachings of the above-identified U.S. Patent are hereby incorporated herein by reference. However, some of the above-recited difficulties remain problematical in that some air gaps have been found to form and thermal transfer into the apparatus can still be excessive.
Consequently, an apparatus for applying gel to a plurality of optical fibers that substantially completely overcomes the above-recited drawbacks is highly desirable and sorely needed in the optical fiber industry.